Spiral Wound Gasket

This invention relates to spiral wound gaskets. A spiral wound gasket is known from, inter alia, GB-A-1026714 and GB-A-1213243. It is constituted by an annulus comprising a narrow strip of a resilient metal wound upon itself to form a spiral, a correspondingly narrow strip of a relatively soft sealant material such as a gasket paper being interleaved between at least the middle turns. The metal strip is usually profiled longitudinally into a shallow `V` or `W` section so as to confer a degree of controlled axial compressibility onto the assembled spiral. The assembled spiral wound gasket is normally used in conjunction with at least an outer guide ring in the form of a metal annulus which is thinner than the uncompressed spiral. This guide ring serves to limit axial compression of the spiral, when in use, to a safe level whereby the elastic limit of the metal is not exceeded. The sealing action of a conventional spiral wound gasket is thus two-fold. Firstly, the edges of the metal strip engage and bite into the confronting flanges between which the spiral is installed. Secondly, the interleaved soft sealant material is compressed and provides a back-up seal. But both aspects rely on the fact that the metal spiral projects a significant amount above the height of the associated guide ring and that the spiral is axially compressed down to the thickness of the guide ring, by the application of very considerable force.

It is an object of the present invention to provide an improved spiral wound gasket which does not relay on the two-fold sealing action just described.

According to the present invention, a spiral wound gasket comprises an annulus constituted by a plurality of superposed turns of a metal strip wound upon itself to form a spiral and, interposed between at least some of said superposed turns, a number of turns of a relatively soft graphite sealant material in strip form, the width of said metal strip being selected so that prior to use it is essentially flush with the surface of an associated guide ring and the width of the strip of the relatively soft sealant material being selected so that prior to use it projects on both sides of the gasket a significant distance from said superposed metal turns axially of said annulus.

In the present context, "essentially flush" means that the metal strip/spiral does not project more than about 0.2 mm from the face of a typical guide ring when installed therein.

"Significant amount" means at least 1 mm, and up to as much as 2 mm relative to the face of the guide ring. The relatively soft graphite sealant material is preferably exfoliated graphite, which has excellent conformability, heat resistance and elastic recovery. Such material is conveniently available in the form of sheet or foil which can readily be slit into strips for interleaving between the metal turns.

It will be appreciated that the underlying philosophy of the spiral wound gasket of this invention is completely at variance with that of a conventional spiral wound gasket, because axial compression of the metal part of the spiral no longer plays a significant part. The superposed metal turns serve primarily as a support for the relatively soft sealant material which projects from the face of the spiral. In fact, it is the sealant material which when compressed develops the primary seal across the face of the gasket. This is to be contrasted with the traditional spiral wound gasket which relies on the metal windings as the primary seal. The spiral turns of steel must project above the guide ring by a significant amount for this to work, but very high forces must be applied to achieve the minimum sealing stress needed. These forces are typically of the order of from 10,000 to 25,000 psi. By contrast, the gaskets of this invention when based on the same steel plus exfoliated/expanded graphite can seal at loadings of less than 5,000 psi, because it is no longer necessary to significantly compress the steel component.

In order that the invention be better understood, embodiments of it will now be described by way of example with reference to the accompanying drawings in which

FIGS. 1 and 2 illustrate the construction of a conventional spiral wound gasket, and

FIGS. 3 and 4 illustrate the construction of a spiral wound gasket according to the present invention.

Referring firstly to FIGS. 1 and 2,

FIG. 1 is a plan view of approximately half of a spiral wound gasket complete with half of its associated guide ring. FIG. 2 is a cross-sectional view taken on line 2--2 of FIG. 1. It is shown slightly enlarged for purposes of illustration. In the Figures, the gasket comprises a plurality of turns of a generally "V" cross-section metal strip. The innermost turns 12 and the outermost turns 13 are free from gasket material. The inner free end 14 is secured by spot welding to the adjacent underlying turn; the outer free end 15 is likewise secured to the adjacent underlying turn, also by spot welding. A plurality of turns 11 of a relatively soft gasket material 11, typically a gasket paper, are interleaved with the intermediate metal turns 10 during winding, as best seen in FIG. 2. The overall spiral structure is thus a laminate sandwiched between plain metal turns.

The radially outer rim of the spiral has a nose region 17 defined by the apex of the "V" cross-section. This enables it to be "snapped into" an outer guide ring 16, the inner edge of which is provided with a machined or pressed groove 18 to receive it.

In use, the guide ring 16 is used to centre the complete gasket within the bolt circle of a flanged pipe joint, in the usual way, (not shown). The bolts are tightened so that the opposed flanges apply pressure to the front and rear faces of the spiral wound annulus. This is then progressively compressed through deformation of the "V" shaped cross-section until the pipe flanges seat onto the faces of the guide ring 16. The loading required to achieve this in a typical, conventional gasket is very high, as is discussed elsewhere in this specification.

Referring now to FIGS. 3 and 4 which illustrate the gaskets of this invention, the construction is superficially similar, FIG. 3 being a plan view similar to FIG. 1 and FIG. 4 being a cross-sectional view taken on line 4--4 of FIG. 3. However, the width of the spiral wound metal strip is considerably reduced, until in its uncompressed state it is not significantly thicker than the guide ring 16. The relatively soft gasket material 11 (in this case exfoliated graphite foil in the form of a strip) is made much wider, so that it projects a significant distance from the metal component turns, 10.

In use, the behaviour of this structure is very different, because there is no significant compression of the metal strip. On bolt-up in a flanged pipe joint, the relatively soft graphite material is completely free to deform/conform to the surface configuration on the confronting flange surfaces without the intervention of the metal part of the spiral. The underlying metal spiral provides a resilient support zone for the gasket material, but because it does not have to be compressed down to the guide ring thickness as was the case with FIGS. 1 and 2, very much lower forces are required to achieve a seal; this is as little as half the bolt loading previously required.

To further illustrate the invention, practical embodiments will now be described with reference to one specific gasket, namely for a bolted flanged joint between two pipes of nominal bore 4 inches and a pressure ring of 150 psi. For this very common application, a conventional spiral wound gasket of the FIG. 1 type would have the parameters given in column 1 below.

The metal winding thickness in the example of FIG. 3 can be determined from the above to be about 9.4% greater than the thickness of the guide ring which is in a range of not greater than 15%. The total gasket thickness is about 40.6% greater than the thickness of the guide ring which is at the upper end of a range of on the order of 25 to 40%.

The second column of figures is for a spiral wound gasket according to the invention and as shown in FIG. 3. Whilst the differences may appear minimal, it should be noted that the exfoliated graphite height above the metal windings is very significant. The FIG. 1 gasket required a bolt loading in excess of 10,000 psi to achieve a seal. By contrast, the FIG. 3 gasket sealed at 5,000 psi or less. From a practical point of view, this is a very significant difference; it is much easier to achieve and at much lower individual bolt loading.

It should be understood that "sealed" in the context of a gasket is a relative term, since in practical gaskets, some very minor leakage is acceptable. Judged on this basis, using pressurised gas as a test medium, the gasket of FIG. 3 exhibited a significantly lower leakage rate than the conventional gasket of FIG. 1, when tested under directly comparable conditions. The good sealing performance of the FIG. 3 gasket was retained even after thermal cycling tests, under which the FIG. 1 gasket might have been expected to show some advantage because of the primary sealing role of the metal windings.